The present invention relates to a hydrogen generating apparatus and, more particularly, to a hydrogen generating apparatus which is suitable for use as a hydrogen generating system or as a mobile electric power generation system when used in conjunction with a fuel cell.
Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Because hydrogen is difficult to store and distribute and because hydrogen has a low volumetric energy density compared to fuels such as gasoline, hydrogen for use in fuel cells will have to be produced at a point near the fuel cell, rather than being produced in a centralized refining facility and distributed like gasoline.
Hydrogen is widely produced for chemical and industrial purposes by converting materials such as hydrocarbons and methanol in a reforming process to produce a synthesis gas.
Synthesis gas is the name generally given to a gaseous mixture principally comprising carbon monoxide and hydrogen, but also possibly containing carbon dioxide and minor amounts of methane and nitrogen. It is used, or is potentially useful, as feedstock in a variety of large-scale chemical processes, for example: the production of methanol, the production of gasoline boiling range hydrocarbons by the Fischer-Tropsch process and the production of ammonia.
Processes for the production of synthesis gas are well known and generally comprise steam reforming, auto-thermal reforming, non-catalytic partial oxidation of light hydrocarbons or non-catalytic partial oxidation of any hydrocarbons. Of these methods, steam reforming is generally used to produce synthesis gas for conversion into ammonia or methanol. In such a process, molecules of hydrocarbons are broken down to produce a hydrogen-rich gas stream.
Modifications of the simple steam reforming processes have been proposed. In particular, there have been suggestions for improving the energy efficiency of such processes in which the heat available from a secondary reforming step is utilized for other purposes within the synthesis gas production process. For example, processes are described in U.S. Pat. No. 4,479,925 in which heat from a secondary reformer is used to provide heat to a primary reformer.
The reforming reaction is expressed by the following formula:
CH4+2H2Oxe2x86x924H2+CO2
where the reaction in the reformer and the reaction in the shift converter are respectively expressed by the following formulae (1) and (2)
CH4+H2Oxe2x86x92CO+3H2
CO+H2Oxe2x86x92H2+CO2
In the conventional hydrogen generating apparatus, an inert gas heated in a reformer is made to flow through a process flow path so as to raise temperatures of the shift converter and the heat exchangers which are downstream from the reformer.
U.S. Pat. No. 5,110,559 discloses an apparatus for hydrogen generation which includes a reformer and a shift converter each incorporating a catalyst wherein, during the start-up of the apparatus, reformer combustion gas is introduced to a shift converter jacket surrounding the shift converter catalyst to heat the shift converter to provide a start-up or temperature rise of the reformer system.
U.S. Pat. No. 4,925,456 discloses a process and an apparatus for the production of synthesis gas which employs a plurality of double pipe heat exchangers for primary reforming in a combined primary and secondary reforming process. The primary reforming zone comprises at least one double-pipe heat exchanger-reactor and the primary reforming catalyst is positioned either in the central core or in the annulus thereof. The invention is further characterized in that the secondary reformer effluent is passed through which ever of the central core or the annulus is not containing the primary reforming catalyst counter-currently to the hydrocarbon-containing gas stream.
U.S. Pat. No. 5,181,937 discloses a system for steam reforming of hydrocarbons into a hydrogen rich gas which comprises a convective reformer device. The convective reformer device comprises an outer shell enclosure for conveying a heating fluid uniformly to and from a core assembly within the outer shell. The core assembly consists of a multiplicity of tubular conducts containing a solid catalyst for contacting a feed mixture open to the path of the feed mixture flow such that the path of the feed mixture flow is separated from the heating fluid flow in the outer shell. In the process, an auto-thermal reformer fully reforms the partially reformed (primary reformer) effluent from the core assembly and supplies heat to the core assembly by passing the fully reformed effluent through the outer shell of the convective reforming device.
Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Because hydrogen is difficult to store and distribute and because hydrogen has a low volumetric energy density compared to fuels such as gasoline, hydrogen for use in fuel cells will have to be produced at a point near the fuel cell, rather than be produced in a centralized refining facility and distributed like gasoline. Polymers with high protonic conductivities are useful as proton exchange membranes (PEM""s) in fuel cells. Among the earliest PEM""s were sulfonated, crosslinked polystyrenes. More recently sulfonated fluorocarbon polymers have been considered. Such PEM""s are described in an article entitled, xe2x80x9cNew Hydrocarbon Proton Exchange Membranes Based on Sulfonated Styrene-Ethylene/Butylene-Styrene Triblock Copolymersxe2x80x9d, by G. E. Wnek, J. N. Rider, J. M. Serpico, A. Einset, S. G. Ehrenberg, and L. Raboin presented in the Electrochemical Society Proceedings (1995), Volume 95-23, pages 247 to 251.
The above processes generally relate to very large industrial facilities and the techniques for integrating the steps of converting the hydrocarbon or alcohol feed stream may not be useful in compact, small-scale hydrogen-producing units to power a transportation vehicle or to supply power to a single residence. One of the problems of large hydrogen facilities is the problem of methane slippage in steam reforming reactors. xe2x80x9cMethane slippagexe2x80x9d is a term used to describe a reduction in the methane conversion across the reforming reactor. Generally, the equilibrium conversion of methane to carbon oxides and hydrogen that is achieved in the reforming reactor increases with temperature. Consequently, a reduction in the reactor outlet temperature corresponds to a lower conversion of methane, or a methane slippage. Methane slippage reduces the overall production of hydrogen and hence the efficiency of the process. Methane slippage can create problems in downstream equipment such as in an oxidation step used to remove trace amounts of carbon monoxide from the hydrogen stream before passing the hydrogen stream to the fuel cell.
It is the objective of this invention to provide a compact apparatus for generating hydrogen from available fuels such as natural gas, hydrocarbons, and alcohols for use in a fuel cell to generate electric power.
It is an objective of this invention to provide an integrated fuel cell and hydrogen production system which is energy and hydrogen efficient.
It is an objective of the present invention to provide an apparatus for the steam reforming of methane which mitigates the methane slippage problem and achieves a more uniform temperature throughout the steam reforming zone.
The steam reforming apparatus of the present invention which has a combustion zone positioned inside a steam reforming zone whereby the steam reforming zone is heated by radiation from the combustion zone and by convection from exhaust gases contacting the inside and the outside of the steam reforming zone provides a simple and efficient system for producing hydrogen from a hydrocarbon or an alcohol stream. By disposing the steam reforming catalyst in a dome shaped, or bell shaped catalyst zone surrounding a combustion zone, the steam reforming zone can be maintained at effective steam reforming conditions which on this small-scale unit minimizes the methane slippage problem of conventional approaches which use a fixed bed reactor or only heat the catalyst from one side. Furthermore, the closed-end top of the bell-shaped catalyst, located above the combustion zone assures that the outlet temperature of the steam reforming reaction zone is maintained at a temperature essentially equal to or greater than the steam reforming reaction zone inlet temperature. The use of a simple burner in the combustion zone with provision to burn both the methane fuel and the anode waste gasxe2x80x94which comprises a significant amount of hydrogenxe2x80x94achieves a better overall energy balance in providing heat to the endothermic steam reforming reaction. The anode waste gas stream is injected into a flame zone provided by the combustion of the methane fuel stream. The further use of the cathode waste gas provides a simplified method of controlling electrical demand induced variations in the combustion zone temperature and the overall energy balance without venting an undesirable steam plume from the fuel cell.
An unexpected benefit of the recycle of a portion of the cathode waste gas to be burned in the combustion zone is that the chance of a plume of condensation forming in the exhaust gas of the process is reduced. A plume of condensation is formed if a warm humid gas is released to the atmosphere at a temperature that is close to the dew point. When the gas meets colder air the moisture is condensed, giving a visible plume, which is undesirable as the public associates such plumes with smoke and pollution.
For an electrical output of about 7 kW, the present invention required a natural gas throughput of about 2.4 normal cubic meters per hour (about 1.4 standard cubic feet per minute) thus providing an overall energy efficiency of about 30 percent.
In one embodiment, the present invention comprises a process for the generation of hydrogen for producing electric power from a fuel cell. A feed stream and a water stream are admixed to provide a feed admixture and the feed admixture is passed to a heat exchange zone to heat the feed admixture by indirect heat exchange to provide a heated admixture. The heated admixture at effective steam reforming conditions is passed to a feed inlet of a steam reforming zone at an inlet temperature to convert the heated feed admixture and produce a steam reforming effluent stream comprising hydrogen and carbon monoxide. The steam reforming zone comprises a compartment provided by a middle space within one or more hollow walls which defines a vessel. The vessel is vertically aligned and has an open-end base and a closed-end top to define a catalyst zone having an inlet into the compartment about the open-end base and an outlet out of the compartment about the closed-end top. The vessel has an inside surface defining an interior, an upper interior adjacent to the closed-end top, and an outside surface. The compartment contains a steam reforming catalyst. A fuel gas mixture is burned in the presence of a first oxygen-containing stream within a combustion zone defined by a combustion tube that contains a flame. The combustion tube extends vertically within the interior of said vessel to provide radiant heat to the steam reforming zone and to produce a fuel exhaust stream. The fuel exhaust stream is circulated downwardly from the upper interior of the vessel over the inside surface of the vessel and upwardly over the outside surface of the vessel to heat the catalyst zone by convection and to maintain the steam reforming catalyst at effective steam reforming conditions. The steam reforming effluent stream is withdrawn from the outlet at an outlet temperature that differs by no more than about 50xc2x0 C. from the inlet temperature.
In another embodiment, the present invention is an apparatus for generating hydrogen for producing electric power from a fuel cell. The apparatus comprises the following elements. A substantially closed outer vessel defines an interior chamber and comprises insulated walls and a base. An inner vessel within the interior chamber has a closed-end top, an open-end bottom and is defined by one or more hollow walls. The middle space within the hollow provides a compartment for retaining catalyst. The inner vessel surrounds a combustion zone. A burner is fixed with respect to the base and is positioned within the combustion zone to provide a flame zone. An anode waste gas conduit is disposed in the combustion zone to provide for the injection of anode waste gas directly into the flame zone. A burner tube is fixed with respect to the base. The burner tube extends vertically above the base to surround the burner. A feed distributor is fixed with respect to the inner vessel and defines a plurality of ports distributed about the open-end bottom for communication with the compartment. A feed conduit is in fluid communication with the feed distributor. A reforming effluent outlet is defined by the closed-end top of the interior vessel and is in fluid communication with the compartment. A fuel exhaust outlet is defined by the top of the outer vessel.